The finding, published today in the journal Cell Metabolism , backs up a mathematical model Hall developed based on decades of studies.

People attempting to lose weight should not be swayed by the "popular theory about how low-carb diets offer a specific metabolic advantage for body fat loss," says Hall.

"Our study has demonstrated that this theory is probably not true."

The study involved 19 participants who were kept as in-patients for two two-week blocks and their diet strictly controlled.

To reduce the variables, the two in-patient blocks were like mirror images: during the first period, 30 per cent of baseline calories were cut through carb restriction alone, while fat and protein intake remained the same.

During the second period the conditions were reversed. Each day, the researchers measured how much fat each participant ate and burned and used this information to calculate the rate of body fat loss.

The experiment showed the reduced fat diet led to body fat loss at a rate of about 89 grams per day whereas an equal calorie reduction through carbohydrate restriction led to only about 53 grams of fat loss per day.

"These are pretty small differences, but highly statistically significant; meaning that they are very unlikely to have occurred by random chance," says Hall.

"So, one of the main results of the study is that not all diet calories are exactly equal when it comes to body fat loss."

He says the dietary claim that low-carbohydrate diets offer a metabolic advantage is not supported by their findings, and points out that calorie use is lower in these diets.

"Whereas the reduced-fat diet led to no significant changes in the total calories burned by the body, the reduced-carb diet led to a significant decrease in overall calorie usage by about 100 kilocalories per day."

Game changer?

Dr Jennie Brand-Miller, a professor of human nutrition at the University of Sydney, says the study is a 'game-changer'.

"You don't need a large number to determine if your mathematical modelling is producing the same findings in 'real life'," she says.

"Their real-life study is highly reliable because they have essentially locked their subjects up and tracked every morsel of food."

Brand-Miller, who is known for her work on the glycemic index — the measure of the body's absorption of carbohydrates — says the study shows a "calorie isn't a calorie after all".

"This study is also a game changer because the science of nutrition in recent years has shown that people lose more weight (faster) on a carbohydrate-restricted diet. So the two sets of findings needed to be reconciled."

But Dr Peter Clifton, a professor of Nutrition at the University of South Australia, says there are no long term findings from the study.

"More changes will occur with time and the differences will disappear," says Clifton.

"In the long term a calorie is a calorie — the body just takes a while to switch its metabolism to match what it burns to what it consumes and five days is not a reflection of what occurs long term."

Indeed, Hall's mathematical modeling predicts that in the long term the body acts to minimise body fat differences between diets that are equal in calories but varying widely in their ratio of carbohydrate to fat.

"Over the long term it's pretty close," says Hall.

The modeling shows perfect adherence to lab diets for six months would only lead to a three kilograms greater fat loss for those on the restricted fat diet.

Ultimately, Hall cautions against using the study to determine what diet to follow.

"Rather than expecting a specific metabolic advantage for choosing one diet over another for losing body fat, it is better to choose a diet that is healthy for you and one that you can stick to for long periods of time, ideally permanently," he says.